The present invention relates to a compound semiconductor device, and more particularly, to a compound semiconductor device specially suited for manufacturing component devices of integrated circuits (IC) and large-scale integrated circuits (LSI) compound semiconductor devices such as field effect transistors (FET), high electron movement transistors (HEMT) and the like.
For fabricating compound semiconductor devices, particularly electronic devices, the epitaxial growth method is generally employed owing to the thin and uniform growth of layers and ease of control of constituent element composition ratio. Also, the molecular beam epitaxial (MBE) growth method is one of the more particularly discussed techniques recently. For example, a device utilizing the MBE growth method and thin layer periodic structure is described in detail by W. T. Tsang in Nikkei Electronics No. 308, 163 (1983).
According to this MBE growth method, the crystal growth speed can be controlled on a single atomic plane level (see J. P. van der Ziel et al., J. Appl. Phys., 48 (1977), p. 3018), and, furthermore, when combined with the reflective electron diffraction method, the composition of one atomic plane can be accurately controlled (see J. H. Neave et al., Appl. Phys. A. 31, 1, 1983).
By employing such an MBE growth method, it is possible to fabricate a high electron mobility transistor (HEMT) like that shown in FIG. 10.
Incidentally, microwave elements using conventional compound semiconductors are disclosed, for example, in the Japanese Unexamined Patent Publication Nos. 4085/1984 and 147169/1983.
In the HEMT structure shown in FIG. 10, a GaAs layer 12 functioning as a buffer layer is formed on a semi-insulating GaAs substrate 11, and an undoped GaAs layer 13 serving as a channel layer is formed thereon. On the undoped GaAs layer 13 is formed an electron supply layer 14 having a high impurity concentration such as n-Ga.sub.x Al.sub.1-x As. In the middle of the electron supply layer 14 there is a layer 15 made of a semiconductor containing p-type impurities at high concentration and possessing a large electrophilic ability, while a gate electrode 16 is disposed on this layer 15. Furthermore, the surface region 17 of the electron supply layer 14 at both sides of the layer 15 is alloyed, and source and drain electrodes 18 are formed thereon.
In the thus composed semiconductor device, when a proper bias voltage is supplied to the gate electrode 16, a two-dimensional electron gas 19 is formed at the channel layer 13 side of the interface of the electron supply layer 14 and the channel layer 13. As a result, many electrons flow within a channel a few tens of angstrom units in thickness near the interface in the channel layer 13 where only few impurity ions are present. Therefore, there is less scattering of impurity ions which is one of the major causes of limiting of the electron mobility, so that a high electron mobility may be realized.
However, when operating such a conventional compound semiconductor device having the typical constitution mentioned above, the electron mobility is extremely dependent on the intensity of the electric field applied to the two-dimensional electron gas. High electron mobility is realized in low electric field application, and conversely the electron mobility is lower in high electric filed application. This phenomenon is described by M. Hnoue et al. in J.J.A.P.22 357 (1983) for example. FIG. 11 shows one of these examples related to the constitution of the above-mentioned microwave element containing GaAs/n-Ga.sub.x Al.sub.1-x As, in which the above phenomenon is expressed by the broken-line curve.
Generally, those phenomena (including inter-valley scattering, impact ionization, or phonon scattering) respectively cause electron scattering in a semiconductor device in high electric field applications. Consequently, those characteristics specified below of the semiconductor crystals used for channel layers of ultra-high-frequency transistors should be improved.
(1) To prevent inter-valley scattering from occurring, a greater value of energy difference .DELTA.E should be provided between valleys of space .kappa..
(2) To prevent impact ionization from occurring, a greater value should be provided for energy gap Eg.
(3) To minimize loss of operating energy of carrier electrons caused by phonon scattering, a lesser value should be provided for effective mass m*.
FIG. 7 shows an example of the energy band structure of GaAs crystals in conjunction with parameters of energy difference .DELTA.E between valleys and energy gap Eg.
Conventionally, GaAs, InP, and In.sub.0.53 Ga.sub.0.47 As respectively make up compound semiconductors for composing channel layers of conventional FET and HEMT. Table 1 represents approximate values of energy difference .DELTA.E between valleys, energy gap Eg, and the effective mass m* of electrons of those compound semiconductors mentioned above. For reference to the later description, data related to InAs and InSb are also shown in Table 1. Note that the energy difference between valleys of In.sub.0.53 Ga.sub.0.47 As is not yet known.
TABLE 1 ______________________________________ Energy gap Energy difference Eg (eV) between valleys (eV) Effective Elements (300k) E.sub..GAMMA.L E.sub..GAMMA.X Mass m* ______________________________________ GaAs 1.428 0.294 0.46 0.065 InP 1.351 0.5 0.85 0.077 InAs 0.356 0.78 1.47 0.027 In.sub.0.53 0.75 -- -- 0.041 Ga.sub.0.47 As InSb 0.18 1.67 2.03 0.016 ______________________________________
As is clear from Table 1, if the channel layer is composed of GaAs compound, since there is a large energy gap Eg, impact ionization rarely takes place in high electric field application. Conversely, due to the small energy difference .DELTA.E between valleys, inter-valley scattering easily takes place. Furthermore, since the effective mass m* of electrons inherent to GaAs is great, a greater amount of operating energy of electrons is lost by phonon scattering, thus preventing electrons from flowing faster. On the other hand, if the channel layer is composed of InP compound, due to substantial values of energy gap Eg, and energy differences E.sub..GAMMA.L, E.sub..GAMMA.X between valleys inherent to InP, neither impact ionization nor inter-valley scattering easily occurs in high electric filed application, thus allowing electrons to flow at a faster speed. Conversely, since the effective mass m* of electrons inherent to InP is great, electrons are presented from flowing at a faster speed in low electric field application. On the other hand, if the channel layer is composed of compound semiconductors made from InAs or InSb, due to the small amount of effective mass m*, electrons flow at an extremely fast speed in low electric field application. Conversely, since the energy gap Eg is small, impact ionization easily occurs in high electric field application. For example, impact ionization occurs in the compound semiconductor made of InAs in about 3.3 KV/cm electric field application. Further, if the channel layer is composed of compound semiconductors made of the mixed crystals of In.sub.0.53 Ga.sub.0.47 As, this mixture exhibits a specific characteristic which is between those of compound semiconductors made of InAs and compound semiconductors made of GaAs. Those features thus far described above are readily identified by referring to FIG. 12 showing the results of tests and theoretical development in conjunction with the dependency of the drift speed of electrons present in compound semiconductors on the intensity of electric field applied thereto. Results of evaluating the characteristic of compound semiconductors made of GaAs are cited by Ruch. J.G. and Kino G.S. in the Phys. Rev., 174, 921 (1969), and by Houston P.A. et al. in Solid State Comm., 20,197 (1977), respectively. Likewise, results of evaluating the characteristic of compound semiconductors made of InP are cited by Nelsen L.D. in the Phys. Lett., A38, 221 (1972) and Boers P.M. in the Electron Lett., 7, 625 (1971), respectively. Results of evaluating the characteristic of compound semiconductors made of InAs are cited by Itoh et al. in the report ED83-77, (1983) presented to Electronic Communication Society of Japan. Covering the results of evaluating the characteristic of compound semiconductors made of InSb, refer to the reports presented by Glicksman M. et al. in the Phys. Rev., 129, 1572 (1963), Neukermans A. et al. in the Appl. Phys. Lett., 17, 102 (1970), and Smith J. et al. in the Appl. Phys. Lett., 37, 797 (1980), respectively. Results of evaluating the characteristic of compound semiconductors made of mixed crystals of In.sub.x Ga.sub.1-x As are cited by Itoh et al. in the report ED83-77, (1983) presented to Electronic Communication Society of Japan.
Generally, when operating either an FET or an HEMT, since either of these is subjected to several kilovolts per centimeter of electric field, any of these transistors having a conventional structure using channel layers made of InAs or InSb which easily allow occurrence of impact ionization in low electric field incurs many disadvantages. Actually, there is no satisfactory FET incorporating channel layers made of either InAs or InSb. In other words, any of these conventional transistors cannot effectively apply advantageous characteristics of InAs or InSb which allows electrons to flow at an extremely fast speed in low electric field application. Although not shown in FIG. 12, those compound semiconductors made of mixed crystals of In.sub.x Ga.sub.1-x As (0&lt;y&lt;1.0) vary their dependency of the electron mobility on the intensity of electric field according to values of x and y. However, the electron mobility in low electric field application of these compound semiconductors is lower than the electron mobility of those which are made of InAs. Yet, the electron mobility of these is also lower than the electron mobility of those compound semiconductors which are made of either GaAs or InP in high electric field application.
In order to manufacture high-performance transistors featuring faster operating speeds and more satisfactory characteristics, such transistors should allow electrons to flow at an extremely fast speed even in low electric field application. Taking this into account, neither GaAs having the low electron mobility in high electric field application nor InP having the low electron mobility in low electric field application is ideally suited for making up channel layers.
A study on a new constitution called a "super-lattice constitution" has been underway, the detail of which is presented by L. Esaki, R. Tsu; IBM J. Res. Develop. (1970) P 61 for example. As shown in FIG. 8, using an adequate growth system like MBE growth method for example, thin-film layers made of compound semiconductors comprising different components like GaAs and AlAs for example are alternately and periodically laminated. By comparative evaluation of bulk crystals, specifically quantized levels are then generated in the thin-film layers in the lower portions of the conduction bank (which are substantially layers A shown in FIG. 8 and are hereinafter called "well layers"). Although FIG. 8 shows only two levels 1 and 2, in actuality, the number and the energy of the level are variably dependent on the thickness LW of the well layer, the thickness LB of the barrier layer corresponding to the layer B shown in FIG. 8, and the difference between the bottoms of the conduction band of layers A and B as the bulk crystals, respectively.
Furthermore, the probability of the presence of electrons in these quantized levels is described by M. Jaros, K.B. Wong; J. Phys. C: Solid State Phys., 17 (1984) L765 for example. Assume that the barrier layer is made of Ga.sub.0.8 Al.sub.0.2 As and the well layer is made of GaAs, then, as shown in FIG. 9-a, only the quantized levels 1 and 2 can be confined inside of the GaAs layer, whereas the high-energy level above level 3 exceeds the barrier height EB of the barrier layer made of Ga.sub.0.8 Al.sub.0.2 As. FIG. 9-b denotes that there is the greater probability of the presence of electrons on the side of the GaAs layer at levels 1 and 2 when the above condition is present. Conversely, there is the greater probability of the presence of electrons inside of the layer made of Ga.sub.0.8 Al.sub.0.2 As. The probability of the presence of electrons is varied by varying the thickness of the barrier layer and the well layer as well as the height of the barrier. More particularly, as is clear from FIG. 9, there is the significantly greater probability of the presence of those electrons having a greater amount of energy than the barrier energy in the Ga.sub.0.8 Al.sub.0.2 As layer than those electrons which move through the GaAs layer. However, since the electron mobility inside of the Ga.sub.x Al.sub.1-x As layer of the periodical thin-film constitution made of mixed crystal of GaAs and Ga.sub.x Al.sub.1-x As is significantly low both in high and low electric field application, when the high electric field is applied, the electron mobility inside of the Ga.sub.x Al.sub.1-x As layer becomes lower than that of the GaAs layer measured in the same intensity of electric field.
Taking the above conditions into account, the present inventor detected that specific energy bank structure identical to those which are shown in FIGS. 8 and 9 could be generated by making up the well layer using either InAs or InSb and the barrier layer by using either InP or GaAs. First, when making up the well layer and the barrier layer by using InAs and InP, respectively, the present inventor detected that the quantized level could be formed inside of the InAs layer. The present inventor also detected that only the low-energy levels 1 and 2 could be confined in the InAs layer, whereas the high-energy level 3 could be raised to a point higher than the barrier height EB between InP and InAs as shown in FIG. 9. As a result, since the effective energy gap Eg inside of the InAs layer is equal to the difference between the quantized level 1 inside of the conduction band and the quantized level 1 inside of the valence band shown in FIG. 8, this energy gap becomes greater than the essential energy gap Eg of the InAs crystal. Consequently, as described earlier, impact ionization cannot easily take place, thus allowing high electric field of greater magnitude to be applied. Alternatively, in the presence of a specific intensity of electric field prior to the occurrence of impact ionization, the probability of the presence of highly energized electrons can significantly grow on the part of the InP layer (refer to the state of level 3 shown in FIG. 9).
Consequently, it is possible for the InAs layer to securely prevent impact ionization from internally taking place. In addition, since those electrons transmitted into the InP layer in high electric field application follow the electric characteristic of the InP layer in high electric field application, as shown in FIG. 12, electrons flow through the InP layer at a speed faster than that of those electrons flowing through other crystals even when high electric field is applied. In other words, in low electric field application, since electrons are confined inside of the InAs layer, electrons flow at a speed faster than those electrons flowing through the GaAs and InP bulk crystals. Yet, since highly-energized electrons flow into the InP layer in high electric field application, electrons are allowed to flow at a speed faster than those which flow through the GaAs crystal.